A nitride semi-conductor light emitting device which emits light in UV-visible region is expected to be widely applied, hygienically, medically, industrially, and has been studied to develop an illumination device, a precision machine or the like, for its compact size and its low electrical consumption. The light emitting device which emits light in specific region including blue-color region has already been developed for practical use.
For development of the nitride semi-conductor light emitting device (including that emitting blue light which is referred to as a blue-color light emitting diode), it is necessary to further improve its light emission efficiency and light output. At present, the semi-conductor light emitting device provides much smaller external quantum efficiency and light output in UV region than blue-color region. The very small external quantum efficiency and light output have been obstacles to practical use of the light emitting device in UV region, and are possibly attributed to very low light emission efficiency in light emission layer (referred to as internal quantum efficiency, hereafter).
Due to transition, point defect, and unintended impurity which are formed in large amount in the light emission layer, the nitride semi-conductor (nitride mixed crystal) has very low internal quantum efficiency. Especially, ternary mixed crystal including Al such as AlGaN has considerably low internal quantum efficiency, for difficulty in growth of the mixed crystal with high quality. Instead, a quaternary mixed crystal AlGaInN has been attracted as a mixed crystal which is less affected by the transition and the point defect than AlGaN, for improving the internal quantum efficiency and light intensity.
A UV LED has been proposed for being utilized as a component of the nitride semi-conductor light emitting device. The UV LED is designed to emit light in 280 to 360 nm at room temperature. The UV LED as the light emitting device has a single quantum well structure formed of AlaGabIncN (0.1≦a≦0.9, 0.02≦c≦0.2, a+b+c=1) with different Al components between well and barrier layers which are respectively grown at growth temperature of 830 to 950° C. A UV LED of JP unexamined patent publication 2005-340856 has a buffer layer, an n-type nitride semi-conductor layer, a light emission layer, a p-type nitride semi-conductor layer, which are respectively formed by metal organic vapor phase epitaxy (MOVPE) method. The growth rate of each AlGaInN layer in the light emission layer is set at 0.12 μm/h.
The Al-containing nitride semi-conductor layer is generally grown at a high temperature of 1000° C. or more. For example, growth temperature is generally set at 1150° C. or more for growth of AlN layer, or 1000 to 1200° C. for growth of AlGaN layer.
In the UV LED including AlGaInN layer of JP Unexamined patent publication 2005-340856, the growth temperature needs to be set at 830 to 950° C. below 1000° C. Under such a low temperature, AlGaInN layer tends to entrap thereinto unwanted oxygen which is generated within a reaction chamber made of quartz or originally included in a raw material or a container of the material, compared to AlGaN layer. Besides, Al needs to be added in large amount in growth of AlGaInN layer for achieving light emission in short wavelength, but Al easily entraps the unwanted oxygen. The entrapped oxygen tends to generate defects, to degenerate layers, and to form undesirable energy levels, thereby decreasing the internal quantum yield of light emission layer.